Nah, it’s a static variable, you only set it once at the first script your game runs when it starts up. Add it somewhere in that script’s Start/Awake/OnEnable/Whatever function.
Unity 3.4 cut it actually, replacing it basically with Screen.sleepTimeout
this gives you much more control, 0 means the same as canDarken = false before, but anything else actually gives you a timeframe till dim, important for pause menues etc